Because activated protein C degrades clotting factors Va and VIIIa, deficiency of protein C predisposes to venous thrombosis.
Protein C is a vitamin K–dependent protein, as are coagulation factors VII, IX, and X, prothrombin, and proteins S and Z. Because activated protein C (APC) degrades factors Va and VIIIa, APC is a natural plasma anticoagulant. Decreased protein C due to genetic or acquired causes promotes venous thrombosis (see Overview of Thrombotic Disorders).
Heterozygous deficiency of plasma protein C has a prevalence of 0.2 to 0.5%; about 75% of people with this defect experience a venous thromboembolism (50% by age 50).
Homozygousor doubly heterozygous deficiency causes neonatal purpura fulminans, ie, severe neonatal disseminated intravascular coagulation (DIC).
Acquired decreases occur in patients with liver disease or DIC and during warfarin therapy.
Diagnosis is based on antigenic and functional plasma assays.
Patients with symptomatic thrombosis require anticoagulation with heparin or low molecular weight heparin, followed by warfarin. Use of the vitamin K antagonist, warfarin, as initial therapy occasionally causes thrombotic skin infarction by lowering vitamin K–dependent protein C levels before a therapeutic decrease has occurred in most vitamin K–dependent clotting factors. It is not known whether the newer oral anticoagulants that inhibit either thrombin (dabigatran) or factor Xa (eg, rivaroxaban, apixaban) can be used in place of other anticoagulants for this disorder.
Neonatal purpura fulminans is fatal without replacement of protein C (using normal plasma or purified concentrate) and anticoagulation with heparin or low molecular weight heparin.
Last full review/revision October 2014 by Joel L. Moake, MD
Content last modified October 2014